Vedantu’s Explanation of Varactor Diode Free PDF
Vedantu is the leader in the field of digital education in India and we are pursuing our mission of providing quality study materials to every section of Indian students which can be accessible to them with ease. Write ups and explanations of various topics can be found on the Website or Mobile application of Vedantu.
What is a Varactor Diode?
A Varactor diode is known as a p-n-junction diode, where the internal capacitance can be altered as a result of the application of reverse bias voltage. Varactor diodes work in the reverse bias condition. They are also known as Varicap diodes or volt Cap diodes indicating varying capacitance. A varactor diode is a voltage-controlled device, i.e., the output of the varactor diode depends on the input voltage supplied, because its function is determined by the applied voltage. The varactor or varicap diodes are primarily used to store charges.
The Diodes are the electronic components used to conduct the flow of electrons in one direction. Diodes are two-terminal devices that are mainly used to protect electronic circuits by controlling voltage and also by transforming AC to DC, but there are many varieties of diodes and they are all having the same basic structure of diode, i.e., sandwiching a p-type material with n-type, also further depending upon the requirement and their characteristic nature they have been classified into rectifier diodes, Zener diodes, Light-emitting diodes(LED’S), Schottky diodes, varactor diodes, etc.
Operation of Varactor Diode
A Varactor diode consists of p-type and n-type semiconductor components respectively. The majority of charge carriers in n-type and p-type semiconductor materials are the electrons and holes respectively. These diodes are designed by sandwiching a p-type material with an n-type material that creates a p-n-junction and the depletion layer is formed at the junction. The width of the depletion region depends on the type of biasing technique used. If the diode is in forward bias condition, the depletion region will be narrow and the conduction current takes place and in reverse biasing, the depletion region will be wide and will block the current.
These varactor diodes are used only in a reverse-biased condition and the current will not flow as the depletion region will block its flow. The primary function of a varactor diode is to store charge, if we use forward biasing the density of charges that can be stored will be zero as they are allowed to flow which is undesirable, thus it is always operated in the reverse bias condition.
Varactor Diode Formulae
The junction capacitance of the varactor diode is inversely proportional to the width of the depletion layer, i.e., if the width of the depletion region is negligible or narrow, the capacitance of the junction will be high. Thus to increase the storage capacity of the varactor diode, the depletion layer must be narrow.
The mathematical equation that gives the junction capacitance of the varactor diode is,
C = εA/W …..(1)
Where,
C- Junction capacitance
ε - Permittivity of the material
A- Area of p-n-junction
W - Width of the depletion region
Further, the relationship between the junction capacitance and the reverse bias voltage is given by the equation:
C = C’K/(V - V’)m
Where,
C - Diode capacitance
C’’ - Diode capacitance before biasing
K - Constant and is equal to 1
V - The barrier voltage
V’ - Reverse bias voltage
m - Constant depends upon which type of semiconducting material is used.
The Quality factor of the Varactor diode is given by,
Q = F/f
Where,
F - The Cut-off frequency or maximum Operating frequency
f - The Operating frequency
Varactor Diode Characteristics
The varactor diodes are manufactured with the intention of resulting in a definite C-V curve that can be obtained by changing the intensity of doping during the process of manufacture. Depending upon the type of doping whether it is linearly doped or non-linearly doped the varactor diodes are classified into the following two types:
Abrupt Varactor diodes
Hyper-Abrupt Varactor diodes
Advantages of Varactor Diodes
The advantages of varactor diodes are as follows:
They are manufactured in such a way that the size of varactor diodes are compact and lightweight.
They are affordable and economical.
They produce relatively less noise in comparison with other diodes.
FAQs on Varactor Diode
1. What is a varactor diode?
A varactor diode, also known as a varicap or tuning diode, is a special type of semiconductor diode designed to function as a voltage-controlled capacitor. It is specifically operated under reverse bias, where its internal capacitance changes predictably in response to the applied voltage.
2. How does a varactor diode work?
A varactor diode's operation relies on its p-n junction and the associated depletion region. When a reverse bias voltage is applied, this region, which is empty of free charge carriers, widens. The p-type and n-type regions act like a capacitor's plates, while the depletion region serves as the dielectric. As the reverse voltage increases, the depletion region widens, effectively increasing the distance between the plates and thus decreasing the diode's capacitance.
3. What does the circuit symbol for a varactor diode represent?
The circuit symbol for a varactor diode combines the standard diode symbol with a capacitor symbol at the cathode end. This unique symbol visually signifies its dual function: it is a diode that is specifically used for its properties as a variable capacitor.
4. What are the most common real-world applications of varactor diodes?
Due to their voltage-variable capacitance, varactor diodes are essential in modern electronics for tuning and frequency control. Key applications include:
- Tuning Circuits: Used in radio receivers, television tuners, and mobile phones to select specific frequencies without mechanical parts.
- Voltage-Controlled Oscillators (VCOs): To precisely control the output frequency of an oscillator using a DC voltage signal.
- Frequency Multipliers and Modulators: Used in communication systems for frequency modulation (FM) and to generate harmonics of a source frequency.
- Phase-Locked Loops (PLLs): As a core component for frequency synthesis and signal tracking.
5. How does a varactor diode differ from a standard p-n junction diode?
The main difference lies in their intended function and design. A standard p-n junction diode is designed for rectification, allowing current to flow in forward bias and blocking it in reverse bias. A varactor diode, however, is specifically designed to operate in the reverse-biased region to utilise its voltage-dependent capacitance. Its doping profile is carefully controlled to maximise this capacitive effect, a property that is incidental and not optimised in a standard diode.
6. Why is the doping profile important in a varactor diode?
The doping profile is crucial because it directly dictates how the capacitance changes with voltage. There are mainly two types:
- An abrupt junction diode has uniform doping, resulting in a capacitance that varies inversely with the square root of the voltage.
- A hyper-abrupt junction diode has a non-uniform doping concentration that is higher near the junction, resulting in a larger and more linear capacitance change with voltage. This makes it ideal for applications that require a wide tuning range.
7. Can a varactor diode be used in forward bias? Why or why not?
No, a varactor diode is not used in forward bias for its primary function. If a forward voltage is applied, it will behave like a regular diode and begin to conduct current, causing the depletion region to collapse. This action would eliminate the variable capacitance effect it is designed for. Its correct operation is strictly within the reverse-biased region, before the breakdown voltage is reached.
8. Why is a varactor diode often preferred over a mechanical variable capacitor in tuning circuits?
A varactor diode offers significant advantages over a mechanical variable capacitor. Being a solid-state device, it is much smaller, lighter, and more cost-effective. It has no moving parts, making it far more reliable and durable. Furthermore, its capacitance can be changed almost instantly by an electronic voltage signal, allowing for fast, precise, and automated tuning, which is not feasible with a manually adjusted mechanical capacitor.
